1. Field of the Invention
This invention generally relates to acoustical transducers and more particularly to acoustical loudspeakers with improved performance characteristics.
2. Description of Related Art
My U.S. Pat. No. 4,595,801 discloses a dual cone loudspeaker with a primary speaker cone similar in function to a conventional dynamic loudspeaker mounted on a frame with a magnet structure. A secondary speaker cone mounts to a subframe on the back of the magnet structure and connects to the primary speaker cone through a rigid coupling device so the primary and secondary speaker cones move in unison. Sound waves from the secondary speaker cone travel through an orifice in a center pole piece of the magnet structure and through a hole in the center of the primary speaker cone radiating in the same direction as sound waves from the primary speaker cone. Consequently for a given excursion of the primary speaker cone my dual cone structure generates a greater sound volume than the primary cone alone by virtue of the simultaneous excursions of both the primary and secondary speaker cones that move a greater air volume for a given speaker cone displacement. As a result this dual-cone loudspeaker is a compound driver or compound acoustic transducer.
More specifically, the compound driver disclosed in my patent includes a primary speaker cone with a frusto-conical form with the center removed that attaches to a bobbin that carries a voice coil. The rigid coupling device includes a center link with radial spokes. The radially outer end of each spoke attaches directly to the secondary speaker cone at the voice coil bobbin.
My U.S. Pat. No. (Ser. No. 09/251,815) filed Feb. 17, 1999 discloses improvements wherein a cylindrical structure attaches to the radially outer end of each spoke and provides a rigid bonding ring that attaches to the voice coil with increased reliability. The entire support structure has an open form and is located centrally of the voice coil. It includes a rigid link that connects to the second speaker cone to produce in the second speaker cone motion corresponding to the voice coil and the first speaker cone.
It has been found that in certain applications, the compound driver described above may be limited in effectiveness by certain aspects of the design. Specifically, the secondary cone drives air through an orifice defined by a vent through the central pole piece of a conventional magnet structure. The size of commercially available voice coils and ring magnets constrains the size of this vent and also limits the ability to construct compound drivers having a large size. Additionally, since the vent has a finite axial length through the magnet, the vent can have the effect of a port. This effect in conjunction with the volume of the compression chamber may create a resonance in certain acoustic applications that could limit the effective bandwidth of the compound driver. Further the spacing between the two loudspeaker cones, controlled by the depth of the magnet structure, may introduce phase cancellation at certain frequencies thereby also limiting bandwidth.
Therefore it is an object of this invention to provide an improved compound driver, particularly in the form of an acoustic transducer such as dual-cone loudspeaker.
It is another object of this invention to provide an improved compound driver, particularly in the form of an acoustic transducer such as dual-cone loudspeaker, that exhibits an improved frequency response.
Still another object of this invention to provide an improved compound driver, particularly in the form of an acoustic transducer such as dual-cone loudspeaker, that minimizes the possibility of phase cancellation.
Yet another object of this invention to provide an improved compound driver, particularly in the form of an acoustic transducer such as dual-cone loudspeaker, that minimizes resonant frequencies over the normal operating frequency range.
In accordance with this invention, an acoustic transducer for generating acoustic waves along a transducer axis includes a frame, a motion generator and open and continuous diaphragms. The frame extends along a transducer axis and has a central opening on the axis. The motion generator connects to the frame and produces an oscillatory output motion along the transducer axis. The open diaphragm has an outer periphery and an inner periphery that defines a central opening on the transducer axis and axially proximate the central frame opening. First and second flexible couplings attach the open diaphragm inner and outer peripheries to the frame generally transverse to the transducer axis and spaced from the motion generator. The continuous diaphragm has an outer periphery, and a third flexible coupling attaches the continuous diaphragm outer periphery to the frame generally transverse to the transducer axis and intermediate of and spaced from the open diaphragm and the motion generator. The continuous diaphragm attaches to the motion generator, and an axially rigid coupling attaches between the continuous and open diaphragms whereby motion of the continuous diaphragm produced by the motion generator produces corresponding motion of the open diaphragm and whereby air intermediate the diaphragms displaced by the continuous diaphragm moves through the central opening of the open diaphragm.
In accordance with another aspect of this invention, a loudspeaker for generating acoustic waves along a speaker axis includes a frame, a magnetic structure with a voice coil and voice coil bobbin, annular and continuous diaphragms and an axially rigid coupling. The frame extends along the loudspeaker axis with a central opening located on the axis. The magnetic structure connects to the frame and includes a magnet that defines an air gap for receiving the voice coil and bobbin whereby electrical signals applied to the voice coil produce oscillatory motion of the voice coil and bobbin in the air gap along the loudspeaker axis. The annular diaphragm has an outer periphery and has an inner periphery that defines a central opening on the axis and axially proximate the central frame opening, and first and second flexible couplings circumscribing the annular diaphragm inner and outer peripheries attach to the frame to position the annular diaphragm transversely to the loudspeaker axis and spaced from the magnet structure. The continuous diaphragm has an outer periphery, and a third flexible coupling circumscribing the continuous diaphragm outer periphery attaches to the frame to position the continuous diaphragm transversely to the loudspeaker axis and intermediate of and spaced from the annular diaphragm and the magnet structure. The continuous diaphragm connects to the bobbin. The axially rigid coupling connects between the continuous and annular diaphragms whereby motion of the continuous diaphragm produced by the bobbin produces corresponding motion of the annular diaphragm and whereby air intermediate the diaphragms displaced by the continuous diaphragm moves through the central opening of the annular diaphragm.
In accordance with yet another aspect of this invention a loudspeaker comprises a frame having a magnetic structure with an air gap and a voice coil for oscillating on a speaker axis through the air gap in response to electrical signals. A first, annular speaker cone has inner and outer peripheries that attach to the frame to be spaced from the magnetic structure. A second speaker cone has an outer periphery that attaches to the frame intermediate and spaced from each of the magnetic structure and the first speaker cone. An axially rigid coupling connects to the spaced first and second speaker cones and to the voice coil whereby oscillation of the voice coil along the speaker axis produces corresponding motion of the first and second speaker cones.